Polysialic acid (PSA) is the result of dynamically regulated posttranslational modification of the neural cell adhesion molecule, NCAM (S. Hoffman and G. M. Edelman, Proc. Natl. Acad. USA 80: 5762-5766 (1983), U. Rutishauser et al., Science 240: 53-57 (1988)). The glycan structure is unusual in vertebrates and has been shown to be attached to the fifth immunoglobulin domain (K. L. Crossin et al., J. Cell Biol. 99: 1848-1855 (1984)). The presence of the large anionic carbohydrate structure that is PSA modulates NCAM binding properties and, by increasing the intercellular space, also influences interactions between other cell surface molecules (S. Hoffman and G. M. Edelman, supra, U. Rutishauser et al., supra, A. Acheson et al., J. Cell Biol. 114: 143-153 (1991), P. Yang et al., J. Cell. Biol. 116: 1487-1496 (1992), P. Doherty et al., Neuron 5: 209-219 (1990)). In the course of embryogenesis the expression of PSA underlies cell type and developmental-specific alterations (G. M. Edelman, Ann. Rev. Cell Biol. 2: 81-116 (1986)) and correlates with stages of cellular motility (G. M. Edelman, supra (1986)), J. Tang et al., Neuron 8: 1031-1044 (1992), L. Landmesser, J. Neurobiol. 23: 1131-1139 (1992)). In the adult, PSA is restricted to regions of permanent neural plasticity and regenerated neural and muscle tissues (G. M. Edelman. supra), R. Martini and M. Schachner, J. Cell Biol. 106: 1735-1746 (1988), J. K. Daniloff et al. J. Cell Biol. 103: 929-945 (1986)). Recent data implicate PSA in spatial learning and memory (H. Cremer et al., Nature 367: 455-459 (1994), H. Tomasiewicz et al., Neuron 11: 1163-1174 (1993)). Of utmost clinical relevance are the observations that polysialylated NCAM forms represent oncodevelopmental antigens in neuroendocrine and hematolymphoid tumors (C. E. C. K. Moolenaar et al., Canc. Res. 50: 1102-1106 (1990), K. Takamatzu et al., Canc. Res. 54: 2598-2603 (1994), P. Komminoth et al., Ann. J. Patho. 139: 297-304 (1991), W. F. Kern et al., Leukemia & Lymphoma 12: 1-10 (1993)). PSA expression enhances the metastatic potential of these tumors and promotes an abnormal localization of metastases (W. F. Kern et al., supra, E. P. Scheidegger et al., J. Lab. Invest. 70: 45-106 (1994)). Studies aimed at enlightening the biosynthetic pathway and the regulation of PSA synthesis suggest the concerted activity of several specific enzymes located within the Golgi apparatus (R. D. McCoy et al., J. Biol. Chem. 260: 12659-12699 (1984), S. Kitazume et al., J. Biol. Chem. 269: 10330-10340 (1994)).
In recent years several mammalian sialyltransferases have been cloned (K. Nara et al., Proc. Natl. Acad. Sci. USA 91: 7952-7956 (1994), H. Kitagawa and J. C. Paulson, J. Biol. Chem. 269: 17875-17878 (1994), K. Sasaki et al., J. Biol. Chem. 269: 15950-15956 (1994) and literature cited therein). All enzymes characterized to date are monosialyltransferases which are specific for both the type of glycosidic linkage and the acceptor structure to which the sialic acid is attached. In fact, the synthesis of PSA in rainbow trout eggs has recently been reported to involve the consecutive activity of several specific enzymes (S. Kitazume, supra). Livingston and Paulson, J. Biol. Chem. 268: 11504-11509 (1993) describe a rat sialyl transferase "STX" which shows homology to the hamster polysialyl transferase (PST) described herein, of about 59%. The human STX sequence is shown in the GenBank.TM. EMBL Data Bank under Accession Number L13445. However, STX does not catalyze any polysialyl reaction.
Three bacterial polysialyl transferases are known (M. Frosch et al., Mol. Microbiol. 5: 1251 (1991); C. Weisgerber et al., Glycobiol. 1: 357 (1991), and S. M. Steenbergen et al., J. Bacteriol. 174: 1099 (1992)). These polysialyl transferases have a substrate and acceptor specificity which is different from the specificity of the enzymes of the invention and do not exhibit any sequence homology with the polysialyl transferases, set forth in the disclosure which follows.
In mammals, particularly humans, PSA is a critical element in the modulation of NCAM binding activities. Thus, there is a need to elucidate, to regulate, and to modulate PSA synthesis, thereby modulating normal and pathological processes which involve, inter alia, NCAM binding.